By disc clean up you mean
I choose everything except compact old files
as I don't use indexing (disabled in services.msc)
or drive compression. Then use
Some programs have large log files also.
Trend Micro's PC-cillin was using half a gig in
Documents and Settings, All Users, Application
Data, Logs. Show hidden files and folders in Control
Panel, Folder Options.
Guess you already use CCleaner.
Might want to read this too...http://support.microsoft.com/kb/315265